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```
``````use crate::iter::{FusedIterator, TrustedLen, UncheckedIterator};
use crate::mem::ManuallyDrop;
use crate::num::NonZero;

/// Creates a new iterator that repeats a single element a given number of times.
///
/// The `repeat_n()` function repeats a single value exactly `n` times.
///
/// This is very similar to using [`repeat()`] with [`Iterator::take()`],
/// but there are two differences:
/// - `repeat_n()` can return the original value, rather than always cloning.
/// - `repeat_n()` produces an [`ExactSizeIterator`].
///
/// [`repeat()`]: crate::iter::repeat
///
/// # Examples
///
/// Basic usage:
///
/// ```
/// use std::iter;
///
/// // four of the number four:
/// let mut four_fours = iter::repeat_n(4, 4);
///
/// assert_eq!(Some(4), four_fours.next());
/// assert_eq!(Some(4), four_fours.next());
/// assert_eq!(Some(4), four_fours.next());
/// assert_eq!(Some(4), four_fours.next());
///
/// // no more fours
/// assert_eq!(None, four_fours.next());
/// ```
///
/// For non-`Copy` types,
///
/// ```
/// use std::iter;
///
/// let v: Vec<i32> = Vec::with_capacity(123);
/// let mut it = iter::repeat_n(v, 5);
///
/// for i in 0..4 {
///     // It starts by cloning things
///     let cloned = it.next().unwrap();
///     assert_eq!(cloned.len(), 0);
///     assert_eq!(cloned.capacity(), 0);
/// }
///
/// // ... but the last item is the original one
/// let last = it.next().unwrap();
/// assert_eq!(last.len(), 0);
/// assert_eq!(last.capacity(), 123);
///
/// // ... and now we're done
/// assert_eq!(None, it.next());
/// ```
#[inline]
#[stable(feature = "iter_repeat_n", since = "1.82.0")]
pub fn repeat_n<T: Clone>(element: T, count: usize) -> RepeatN<T> {
let mut element = ManuallyDrop::new(element);

if count == 0 {
// SAFETY: we definitely haven't dropped it yet, since we only just got
// passed it in, and because the count is zero the instance we're about
// to create won't drop it, so to avoid leaking we need to now.
unsafe { ManuallyDrop::drop(&mut element) };
}

RepeatN { element, count }
}

/// An iterator that repeats an element an exact number of times.
///
/// This `struct` is created by the [`repeat_n()`] function.
/// See its documentation for more.
#[derive(Clone, Debug)]
#[stable(feature = "iter_repeat_n", since = "1.82.0")]
pub struct RepeatN<A> {
count: usize,
// Invariant: has been dropped iff count == 0.
element: ManuallyDrop<A>,
}

impl<A> RepeatN<A> {
/// If we haven't already dropped the element, return it in an option.
///
/// Clears the count so it won't be dropped again later.
#[inline]
fn take_element(&mut self) -> Option<A> {
if self.count > 0 {
self.count = 0;
// SAFETY: We just set count to zero so it won't be dropped again,
// and it used to be non-zero so it hasn't already been dropped.
unsafe { Some(ManuallyDrop::take(&mut self.element)) }
} else {
None
}
}
}

#[stable(feature = "iter_repeat_n", since = "1.82.0")]
impl<A> Drop for RepeatN<A> {
fn drop(&mut self) {
self.take_element();
}
}

#[stable(feature = "iter_repeat_n", since = "1.82.0")]
impl<A: Clone> Iterator for RepeatN<A> {
type Item = A;

#[inline]
fn next(&mut self) -> Option<A> {
if self.count > 0 {
// SAFETY: Just checked it's not empty
unsafe { Some(self.next_unchecked()) }
} else {
None
}
}

#[inline]
fn size_hint(&self) -> (usize, Option<usize>) {
let len = self.len();
(len, Some(len))
}

#[inline]
fn advance_by(&mut self, skip: usize) -> Result<(), NonZero<usize>> {
let len = self.count;

if skip >= len {
self.take_element();
}

if skip > len {
// SAFETY: we just checked that the difference is positive
Err(unsafe { NonZero::new_unchecked(skip - len) })
} else {
self.count = len - skip;
Ok(())
}
}

#[inline]
fn last(mut self) -> Option<A> {
self.take_element()
}

#[inline]
fn count(self) -> usize {
self.len()
}
}

#[stable(feature = "iter_repeat_n", since = "1.82.0")]
impl<A: Clone> ExactSizeIterator for RepeatN<A> {
fn len(&self) -> usize {
self.count
}
}

#[stable(feature = "iter_repeat_n", since = "1.82.0")]
impl<A: Clone> DoubleEndedIterator for RepeatN<A> {
#[inline]
fn next_back(&mut self) -> Option<A> {
self.next()
}

#[inline]
fn advance_back_by(&mut self, n: usize) -> Result<(), NonZero<usize>> {
}

#[inline]
fn nth_back(&mut self, n: usize) -> Option<A> {
self.nth(n)
}
}

#[stable(feature = "iter_repeat_n", since = "1.82.0")]
impl<A: Clone> FusedIterator for RepeatN<A> {}

#[unstable(feature = "trusted_len", issue = "37572")]
unsafe impl<A: Clone> TrustedLen for RepeatN<A> {}
#[stable(feature = "iter_repeat_n", since = "1.82.0")]
impl<A: Clone> UncheckedIterator for RepeatN<A> {
#[inline]
unsafe fn next_unchecked(&mut self) -> Self::Item {
// SAFETY: The caller promised the iterator isn't empty
self.count = unsafe { self.count.unchecked_sub(1) };
if self.count == 0 {
// SAFETY: the check above ensured that the count used to be non-zero,
// so element hasn't been dropped yet, and we just lowered the count to
// zero so it won't be dropped later, and thus it's okay to take it here.
unsafe { ManuallyDrop::take(&mut self.element) }
} else {
A::clone(&self.element)
}
}
}
``````